1. Field of the Invention
This invention relates to apparatus and a method for the treatment of natural gas and more particularly to the separation of ethane and higher boiling point hydrocarbons from natural gas.
2. Description of the Prior Art
Natural gas produced from gas wells or associated with crude oil produced in oil wells is principally methane but usually contains ethane and hydrocarbons such as propane, butane, etc. having a higher boiling point than ethane. It is desirable to remove the ethane and higher boiling point hydrocarbons from natural gas before delivery of the natural gas to pipelines to recover low boiling point hydrocarbon liquids having a high value and to remove condensable materials from the gas delivered to the pipeline. A substantial quantity of liquids, particularly propane and higher boiling point hydrocarbons, in the natural gas delivered to pipelines can interfere with pumping the gas through the pipeline. Moreover, if there is an appreciable quantity of moisture in the natural gas, solid hydrates will form which may plug pipeline equipment.
The most modern plants used to separate ethane and other higher boiling point hydrocarbons from natural gas utilize a cryogenic process in which the natural gas is cooled to a temperature cool enough to cause condensation of ethane and higher boiling point hydrocarbons. Such plants have required high capital expenditures that could be justified only if there were large quantities of gas to be treated. A recent survey indicated that over 40 percent of the plants for condensation of natural gas liquids from natural gas had capacities for treating in excess of 50,000,000 cu.ft./day of natural gas. Such plants are not installed until it has been proven that there is a substantial gas reservoir that will supply adequate gas for amortization of the large investment for the natural gas liquids plant and to justify a liquids product pipeline to take the liquid hydrocarbon product separated from the gas.
Now that most of the more promising structures have been drilled for oil or gas many of the discovery wells produce relatively small amounts of gas, for example, 1,000,000 to 10,000,000 cu.ft./day. Formerly, if the well were an oil well, the gas would be flared but regulations now prevent flaring. If a gas pipeline is nearby, the associated gas may be delivered to the pipeline without treatment. The added value of the ethane and higher boiling point hydrocarbons in the natural gas is lost to the well owner. Sometimes the well cannot be produced until there is sufficient development of the field to justify one of the larger capacity natural gas liquids plants. The delay in production from the well may have such an adverse effect on the return on investment that the field is not developed.
It is desirable to provide apparatus and a process for the separation from natural gas of ethane and higher boiling point hydrocarbons that will require low capital investment and that are capable of treating small gas flows efficiently. The plant should be portable to permit its easy and rapid transfer from a well after there has been sufficient development of the field around the well to justify a conventional natural gas liquids plant. It is contemplated that the portable plant will then be moved to a new discovery well for recovery of natural gas liquids from the production from that well, and for that reason it is desirable that the plant be operable unattended over a wide range of flow rates.
U.S. Pat. No. 3,292,380 of Bucklin describes apparatus for treating natural gas to recover natural gas liquids. In the apparatus, the gas is cooled to condense some of the higher boiling point hydrocarbons in the gas and the uncondensed vapors expanded in a turbine to reduce the temperature of the vapors to a range at which ethane is condensed. The vapors are delivered into a fractionating tower for separation of methane as an overhead gaseous product from the other hydrocarbons in the natural gas. The hydrocarbons condensed during the cooling are separated from the vapors, flashed to a lower pressure and the liquid resulting from the flashing delivered into the fractionating tower at a midpoint thereof. Power from the turbine is used to compress gas withdrawn from the top of the fractionating tower and from the flash drum to pipeline pressures. The process described in the Bludworth U.S. Pat. No. 3,292,381 is very similar to the process described in U.S. Pat. No. 3,292,380. Other patents describing cryogenic processes for the recovery of natural gas liquids from natural gas are Streich U.S. Pat. No. 3,494,751; Streich U.S. Pat. No. 3,596,472; Swenson et al U.S. Pat. No. 3,393,527 and DiNapoli U.S. Pat. No. 3,359,473. In Streich U.S. Pat. No. 3,596,472, a cascade-type refrigeration process is used to develop temperatures low enough to produce liquefied natural gas.